.\" -*- nroff -*-
.\" Copyright 2010 Cisco Systems, Inc.  All rights reserved.
.\" Copyright 2006-2008 Sun Microsystems, Inc.
.\" Copyright (c) 1996 Thinking Machines Corporation
.\" $COPYRIGHT$
.TH MPI_Graph_neighbors 3 "Jun 10, 2020" "4.0.4" "Open MPI"
.SH NAME
\fBMPI_Graph_neighbors \fP \- Returns the neighbors of a node associated with a graph topology.

.SH SYNTAX
.ft R
.SH C Syntax
.nf
#include <mpi.h>
int MPI_Graph_neighbors(MPI_Comm \fIcomm\fP, int\fI rank\fP, int\fI maxneighbors\fP,
	int\fI neighbors\fP[])

.fi
.SH INPUT PARAMETERS
.ft R
.TP 1i
comm
Communicator with graph topology (handle).
.TP 1i
rank
Rank of process in group of comm (integer).
.TP 1i
maxneighbors
Size of array neighbors (integer).

.SH OUTPUT PARAMETERS
.ft R
.TP 1i
neighbors
Ranks of processes that are neighbors to specified process (array of integers).
.ft R
.TP 1i
IERROR
Fortran only: Error status (integer).

.SH DESCRIPTION
.ft R
\fBExample:\fP  Suppose that comm is a communicator with a shuffle-exchange
topology. The group has 2n members. Each process is labeled by a(1),\ ..., a(n) with a(i) E{0,1}, and has three neighbors: exchange (a(1),\ ..., a(n) = a(1),\ ..., a(n-1), a(n) (a = 1 - a), shuffle (a(1),\ ..., a(n)) = a(2),\ ..., a(n), a(1), and unshuffle (a(1),\ ..., a(n)) = a(n), a(1),\ ..., a(n-1). The graph adjacency list is illustrated below for n=3.
.sp
.nf
        		exchange		shuffle		unshuffle
    node		neighbors(1)	neighbors(2)	neighbors(3)
    0(000)	    1		    0		    0
    1(001)	    0		    2		    4
    2(010)	    3		    4		    1
    3(011)	    2		    6		    5
    4(100)	    5		    1		    2
    5(101)	    4		    3		    6
    6(110)	    7		    5		    3
    7(111)	    6		    7		    7
.fi
.sp
Suppose that the communicator comm has this topology associated with it. The following code fragment cycles through the three types of neighbors and performs an appropriate permutation for each.
.sp
.nf
C  assume: each process has stored a real number A.
C  extract neighborhood information
      CALL MPI_COMM_RANK(comm, myrank, ierr)
      CALL MPI_GRAPH_NEIGHBORS(comm, myrank, 3, neighbors, ierr)
C  perform exchange permutation
      CALL MPI_SENDRECV_REPLACE(A, 1, MPI_REAL, neighbors(1), 0,
     +     neighbors(1), 0, comm, status, ierr)
C  perform shuffle permutation
      CALL MPI_SENDRECV_REPLACE(A, 1, MPI_REAL, neighbors(2), 0,
     +     neighbors(3), 0, comm, status, ierr)
C  perform unshuffle permutation
      CALL MPI_SENDRECV_REPLACE(A, 1, MPI_REAL, neighbors(3), 0,
     +     neighbors(2), 0, comm, status, ierr)

.fi
.SH ERRORS
Almost all MPI routines return an error value; C routines as the value of the function and Fortran routines in the last argument. C++ functions do not return errors. If the default error handler is set to MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception mechanism will be used to throw an MPI::Exception object.
.sp
Before the error value is returned, the current MPI error handler is
called. By default, this error handler aborts the MPI job, except for I/O function errors. The error handler may be changed with MPI_Comm_set_errhandler; the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarantee that an MPI program can continue past an error.

.SH SEE ALSO
.ft R
.sp
MPI_Graph_neighbors_count

